Organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic solar cells (also referred to as organic photovoltaics or OPVs) based on π-conjugated materials have been extensively studied since the early 1990s because of their low-cost processing, relatively simple packaging, and compatibility with flexible substrates (Friend et al., Nature 1999, 397, 121; Katz et al., Acc. Chem. Res. 2001, 34, 359; and Brabec et al., Adv. Funct. Mater. 2001, 11, 15).
More specifically, OPVs come with the promise of efficient conversion of sunlight into direct usable electrical energy at a much lower cost than the traditional silicon based solar cells. It is known that for efficient energy conversion in OPVs, a mixture of at least two (sometimes more) organic materials are needed: at least one material that can act as an electron donor (p-type) and at least one material that can act as electron acceptor (n-type).
Materials research in OPVs since the mid-1990s has focused primarily on the development of donor materials. As a result, there are many more organic donor materials commercially available than organic acceptor materials. For example, the most widely investigated organic semiconductors are donor materials based on aromatic amines and thiophene materials (Katz et al., Acc. Chem. Res. 2001, 34, 359; Chan and Ng, Prog. Polym. Sci. 1998, 23, 1167; and Thelakkat, Macromol. Mater. Eng. 2002, 287, 442).
In contrast, research in electron acceptors has substantially lagged behind. Work in this area has primarily focussed on perylene and fullerene materials, which in general are relatively difficult to work with in terms of synthesis of the materials. The field of high-performance organic electronic devices will advance significantly with the development of new organic n-type semiconductors with high electron mobilities and controllable HOMO and LUMO energy levels (Greenham, et al., Nature 1993, 365, 628; Kulkarni et al., Chem. Mater. 2004, 16, 4556; and Hughes and Bryce, J. Mater. Chem. 2005, 15, 94).
For example, OPVs incorporating heterojunctions of p- and n-type conjugated materials demonstrate much better performance than devices incorporating only a single type of material, since the heterojunction promotes dissociation of photogenerated excitons into free charge carriers that in turn create the desired photovoltaic effect to generate electricity (Brabec et al., Adv. Funct. Mater. 2001, 11, 15; Halls et al., Nature 1995, 376, 498; Hoppe and Sariciftci, J. Mater. Chem. 2006, 16, 45; and Kietzke et al., Macromolecules 2006, 39, 4018).
Currently, perylenes and fullerenes are the dominant n-type material used in OPVs. The chemistry of these compounds is relatively well known, with little room for new developments. Furthermore, production of these materials tends to be very expensive, especially in the case of fullerene based materials. Perylenes are typically insoluble, meaning that often only vacuum deposition of these compounds is possible. Since the mid-1990s, fullerene compounds have been optimized for use in solution processable organic solar cells, providing power conversion efficiencies in the range of 2-5% when combined with selected commercial donor materials. Despite this, fullerenes tend to have low absorption coefficients in the visible range, are difficult to synthesize, and have low open circuit voltage in blend devices.
Accordingly, there is a need for production of alternative n-type organic materials useful for production of efficient electronic devices, including OPVs.